1. Technical Field
The present invention relates generally to a wireless communication system, a wireless communication control apparatus and a wireless communication control method, and a computer program which realize mutual communication between a plurality of wireless stations and, more particularly, to a wireless communication system, a wireless communication control apparatus and a wireless communication control method, and a computer program which realize the construction of a network under the control of a particular control station.
To be more specific, the present invention relates generally to a wireless communication system in which a plurality of wireless networks exist at the same time, a wireless communication control apparatus and a wireless communication control method for controlling the communication operation in each of a plurality of contending wireless networks, and a computer program. Still more specific, the present invention relates to a wireless communication system in which a plurality of wireless networks contending with each other at a same frequency band exist at the same time, a wireless communication control apparatus and a wireless communication control method for controlling the communication operation in each of a plurality of wireless networks contending with each other at a same frequency band, and a computer program. (It should be noted that the “same frequency band” referred to herein includes UWB (Ultra Wide Band) wireless communications technology by which data are transferred as spread over ultra wide band.)
2. Background Art
Configuring a LAN (Local Area Network) by interconnecting a plurality of computers allows the sharing of information such as files and data, the sharing of peripheral devices such as printers, and the exchanging of information such as electronic mail and content data.
Conventionally, it is a general practice to perform LAN connections in a wired manner based on optical fibers, coaxial cables, or twisted-pair cables. This requires line laying work, making it difficult to construct networks with ease and making cable wiring complicated. After the construction of a LAN, the moving range of apparatuses is limited by the cable lengths, thereby making it inconvenient to use the constructed system. To free the users from the conventional wired LAN systems, wireless LANs have been attracting attention of those who are concerned. According to wireless LANs, most of, the cables laid in offices and other work spaces can be left out, thereby making it comparatively easy to move communication terminals personal computers (PCs).
Recently, because of their enhanced transmission speeds and reduced costs, wireless LAN systems have been increasingly in high demand. Especially, it is a recent trend that the communication of information is performed by constructing small-scale wireless networks among plural personal electronic devices, so that researches are being made into the introduction of a personal area network (PAN). For example, PAN specifies different wireless combination systems based on 2.4 GHz and 5 GHz bands, which do not require the license from authorities concerned.
For example, the IEEE 802.15.3 standardizes personal area networks having transfer rates higher than 20 Mbps. This IEEE section is mainly promoting the standardization based on the PHY layer mainly using signals of 2.4 GHz band.
In these wireless personal networks, one wireless communication apparatus operates as a control station called a “coordinator” around which a personal area network is constructed within a range of about 10 meters. The coordinator transmits a beacon signal at a predetermined period and this beacon period is specified as a transmission frame period. At every transmission frame period, the time slots to be used by the wireless communication apparatuses of the wireless personal network are assigned.
For the time slot assigning, methods called “guaranteed time slot (GTS)” and “dynamic TDMA (Time Division Multiple Access)” for example are used in which a communication method is assumed for dynamically assigning transmission time slots while guaranteeing a predetermined transmission capacity.
For example, the MAC layer standardized by the IEEE 802.15.3 prepares the contention access period (CAP) and the contention free period (CFP). For asynchronous communication, the contention access period is used to exchange short data and command information. For stream communication, dynamic time slot assignment is performed by the guaranteed time slot (GTS) in the contention free period, thereby performing guaranteed time slot transmission.
The MAC layer portion standardized by the IEEE 802.15.3 is specified so that it is applicable as the standard specifications of the PHY layer in addition to the PHY layer based on 2.4 GHz band signal. Also, the standardizing activities have begun for the application of the PHY layer standardized by the IEEE 802.1.5.3 to other PHY layers than the PHY layer based on 2.4 GHz band signal.
Recently, wireless LAN (Local Area Network) systems based on SS (Spread Spectrum) technologies have been put into practical use. Also recently proposed is the UWB (Ultra Wide Band) transmission scheme based on the SS intended for applications such as PAN.
The DS (Direct Spread) scheme, which is one of SS schemes, multiplies an information signal by a random code sequence called PN (Pseudo Noise) to spread the occupied band and transmits the resultant signal on the transmitting side; on the receiving side, the received spread information signal is multiplied by the PN code for de-spread to reproduce the information signal. In the UWB scheme, this information signal spread rate is maximized by which a high-speed data transmission is realized by performing to transmit/receive data such that the data is spread over an ultra high frequency band of 2 GHz to 6 GHz for example.
In the UWB scheme, an information signal is configured by use of impulse signal trains each having a period of as short as several 100 picoseconds and this signal train is transferred. Its occupied band width is in GHz order so that the value obtained by dividing the occupied band width by its center frequency (for example, 1 GHz to 10 GHz) becomes about 1, which is a ultra wide band as compared with the band widths commonly used in wireless LANs based on so-called W-CDMA, cdma 2000, SS (Spread Spectrum), and OFDM (Orthogonal Frequency Division Multiplexing) schemes.
Referring to FIG. 20, there is shown an example of the data transmission based on the UWB scheme. Input information 901 is spread by a spread sequence 902. In some systems based on UWB, the multiplication of this spread sequence is left out.
An information signal 903 resulted from the spectrum spread is demodulated by use of the impulse signal (wavelet pulse) of the UWB scheme (905). This modulation is based on PPM (Pulse Position Modulation), biphase modulation, or amplitude modulation for example.
The impulse signal used on the UWB scheme is an extremely thin pulse, so that the UWB scheme uses extremely wide band in terms of frequency spectrum. Consequently, the inputted information signal has only a power that is lower than the noise level in each frequency area.
A received signal 905 is mixed with noise, but it can be detected by computing the correlation value between the receive signal and the impulse signal. In addition, because signal spread is performed in many systems, many impulse signals are transmitted for one bit of transmit information. Hence, a receive correlation value 907 of each impulse signal may be integrated by the length of spread sequence (908), thereby further facilitating the detection of the transmit signal.
The signal spread by the UWB transmission scheme has only a power lower than the noise level in each frequency area, so that it is comparatively easy for each communication system based on the UWB transmission scheme to coexist with communication systems based on other communication schemes.
When a communication environment is considered in which, with the recent popularization of information devices such as personal computers (PCs) and installation of many various devices in offices, these devices are interconnected by two or more wireless networks packed in a narrow work environment, thus causing plural wireless networks coexist in same frequency band. The “same frequency band” herein includes the UWB wireless communication scheme, which transfers data by spreading them over a very wide frequency band.
The PHY layer specifications using the 2.4 GHz band signal standardized by the above-mentioned IEEE 802.15.3 must consider the coexistence with other plural wireless communication systems existing in the same frequency band.
Especially, in the case of the UWB wireless communication network, data are transferred by spreading them over an extremely wide band, so that it is highly possible for this network to contend with adjacent wireless communication networks.
On the other hand, an impulse signal train used in the UWB wireless communication scheme has no particular frequency carrier, so that it is difficult to perform a carrier sense operation. Therefore, if the UWB wireless communication scheme is applied as the PHY layer of the IEEE 802.15.3, the carrier sense standardized by this section cannot be used for access control because there is no particularly carrier signal, thereby having to resort to the access control based on time division multiplexing.
If a small-scale wireless network system such as PAN is put into consideration, each network (or base station) is not always fixed, so that, if a new network is constructed in the same space or a network is moved to the same space from another space for example, the problems of the contention between networks and the dynamic assignment of time slots (or resources) must be solved.
For example, Japanese Patent Laid-open No. 2000-299670 assigned to the applicant hereof discloses a network system, by assigning at least one of plural divided slots to a control slot, which transmits the information suitable for the network status and the contents of information to be transmitted.
The above-mentioned disclosed network system is based on a method in which each terminal station reports interfered slots to the control station and the control station circumvents these interfered slots to use the network.
However, this method in which each terminal station reports to the control station requires a means for reporting to the control station from time to time, thereby presenting a problem of increasing the frequency of reporting if there are plural adjacent networks.
In addition, the above-mentioned method, because the usage status of each slot is known by detecting a predetermined synchronous signal, all frame structures used by other networks cannot be understood.
Japanese Patent Publication No. 2,660,189 discloses a method and apparatus capable of dynamically assign a bandwidth to plural cells within a cellular network. However, according to this application, the disclosed method and apparatus must organize plural cell groups called “super cells” such that the interference between cells is minimized. The request for channel band widths is performed in accordance with the user request in each cell, namely, the request from the mobile stations (MS) belonging to each base station (BS). In other words, if plural networks coexist in the same space, the request for channel bandwidths has no relationship with how to solve the problem of contention between the base stations. Further, the above-mentioned Japanese patent publication does not solve the problems of the contention between networks and dynamic assignment of time slots (or resources) in the case where a new network is constructed in the same space or a network moves to this same space from another space.
Japanese Translations of PCT for Patent No. 2001-518766 (WO99/17575) discloses a method in which, by dynamically dividing the data transmitting resources by plural networks, each network divides the assigned resources among its users in accordance with its own channel assigning method. This application is based on the concept in which the operators mutually accommodate the plural frequency channels existing in the same space to operate the network, thereby realizing the dynamic assignment of frequency channels by borrowing the (frequency) bands of different operators in a cellular network.
However, the above-mentioned application treats the problem of dynamic assignment of the transmitting resources between the base stations which are arranged in a fixed manner, so that it is presumed that a minimum time slot be prepared for each network (or base station) from the very start. Further, each network (or base station) arranged in a fixed manner requires the minimum resource assignment for accommodating the users, so that there is no state in which no resource is assigned to the network. In other words, the above-mentioned application does not propose any scheme, procedure, and method for a network newly constructed in the same space or moving thereto from another space to get resources from the state in which no resource is assigned to the network. The above-mentioned application does not refer to any scheme, procedure, and method for a network already constructed in a space to assign resources to a network that newly appears in the same space.